It is known from prior art, for example from U.S. Pat. No. 4,553,124, to install strain gauges inside bolts in order to sense strain. However, these known strain gauges are complicated and thereby expensive. Another drawback is that they are temperature dependant.
JP-A-11183280 describes a bolt fastening or tightening method in which the elongation of the bolt is being measured while tightening of the bolt is performed. The fastening or tightening operation is terminated when the elongation reaches a predetermined value. The method used to measure the elongation includes a step of operating a laser. In this known method, the elongation of the bolt is monitored when the bolt is being fastened/tightened, but when the tightening is terminated no further information of the elongation is available. Thus, it is impossible to obtain information if the bolt is loosened.
Further background art is disclosed in US2010/0050778 and US2007/0204699.
From the above it is understood that there is room for improvements.